There exist a variety of optical techniques for detecting vibration in an object of interest. Several techniques, based upon time domain processing, use homodyne or heterodyne interferometry, and have been applied in several manners to the field of laser ultrasonics. According to such techniques, a photodetector and signal processing apparatus are used to record vibration displacement amplitudes in the form of interference recorded at the photodetector. An interferometer is an instrument that is well recognized in the art in which light from a source is split into two or more beams which are subsequently reunited after traveling over different paths and which display individual interference patterns.
The above-noted techniques typically use pulsed time domain techniques and wide bandwidths, and are usually employed to obtain real-time surface motion under various conditions. Frequency domain continuous measurements are useful in particular applications, such as when performing structural analysis, because frequency domain measurements record the randomly or continuously excited vibrational spectrum of the entire object being analyzed.
While the above-described techniques have provided some degree of success, there exist several shortcomings needing resolution. For example, for the case of time domain-based analysis techniques, a significant signal-to-noise ratio improvement can be gained by employing a reduced bandwidth of the measurement as compared to the time domain methods, but at the expense of measurement speed.
An additional shortcoming of the above-described optical approaches to detecting vibration has been the sensitivity of such methods to speckle reflections from the specimen surface. A speckle reflection is a light phenomenon which accompanies the scattering of coherent light from a surface. Such a phenomenon may occur, for example, when a light source such as a laser hits a roughened or inhomogeneous medium and generates a random-intensity distribution of light that gives the surface or medium a granular appearance. Speckle reflection can be corrected by limiting detection to a single speckle, or by using self-beating interferometers such as a Fabry-Perot interferometer.
Some prior art techniques use a photorefractive effect in optically nonlinear materials to detect vibration. Such a method provides an active mechanism for compensation using the spatial and temporal characteristics of photorefractivity. Accordingly, several optical frequency domain measurement methods have been proposed using photorefractive, two- and four-wave mixing, in selected materials. These techniques provide a response that is substantially a nonlinear function of the specimen vibration displacement amplitude (typically, a Bessel function having order zero) and often do not provide a measure of the vibration phase.
A major shortcoming of the prior art techniques is their inability to image more than one surface point at a time, wherein only point vibration detection is possible. Furthermore, prior art techniques require a significant amount of computer post processing of scanned point measurements data to produce an image of the vibration displacement of the specimen surface.
Therefore, it is desirable to provide a vibration detection assembly and method which retains the individual benefits derived from the prior art techniques and devices, while avoiding the detriments individually associated therewith, and with a simplified design and implementation.
The object of the present invention is to provide a vastly improved vibration detection assembly and method particularly suited for use with diffusely reflecting surfaces and having a greatly enhanced sensitivity, linear output for small vibration amplitudes (proportional to Bessel function of order one), while simultaneously providing an image of the vibration amplitude over the surface of the specimen while enabling surface imaging.